Conventionally, a particle analyzer which uses an analysis method called “flow cytometry” is used as a means for counting blood cells in blood.
In the process of counting erythrocytes in blood, collected blood is diluted with physiological saline to prepare a blood sample, and the blood sample is introduced into a particle analyzer, in which particles having a specific volume range (e.g., 36-60 femtoliter (fL=10−15 liter)) are then counted one by one. At this time, leukocytes having a volume almost equal to that of erythrocytes are also counted, but the error caused by counting leukocytes is negligible because the concentration of leukocytes in blood is generally about 0.1-0.2% based on the concentration of erythrocytes.
In contrast, in the process of counting leukocytes in blood, collected blood is diluted with a hemolytic reagent to prepare a blood sample, and the blood sample is introduced into a particle analyzer, in which particles having a specific volume range (e.g., about 25 to 450 fL) are then counted one by one. The hemolytic reagent contains a component that breaks the membrane of blood cells. Because erythrocytes have no nucleus, when the membrane of the erythrocytes is broken, hemoglobin is released and the erythrocytes lose their shape or size, and thus are lysed. On the other hand, because the leukocytes have a nucleus, these cells are lysed to leave bare nuclei (particles) which are then dispersed in the blood sample. Accordingly, leukocytes can be counted by introducing a blood sample containing lysed erythrocytes into a particle analyzer and counting particles having a specific volume range in the particle analyzer.
Methods of collecting blood from a living body in blood testing include two methods: a method of collecting venous blood from a subcutaneous vein; and a method of collecting capillary blood from a fingertip or an earlobe.
In the case in which a blood sample obtained by diluting venous blood with a hemolytic reagent is analyzed in a particle analyzer, as shown in the leukocyte volume histogram of FIG. 11, the noise derived from the debris of lysed platelets or erythrocytes (ghost “G”) and the peak derived from leukocytes (leukocyte “W”) are clearly distinguished from each other. Thus, leukocytes can be counted with relatively high accuracy.
Because collection of venous blood is performed by inserting a syringe needle into a subcutaneous vein and collecting blood directly from the blood vessel, the aggregation of platelets immediately after blood collection is not generally admitted. In addition, collected venous blood is diluted with a hemolytic reagent containing ethylenediamine tetraacetate as an anti-aggregation agent (platelet anti-aggregation agent), and thus a means for inhibiting the aggregation of platelets in a blood sample is established (see, for example, Patent Document 1).